In fiber-optic communications, WDM is a technology which multiplexes multiple optical carrier signals on a single optical fiber by using different wavelengths (colours) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to enabling bidirectional communications over one strand of fiber. A WDM system uses a multiplexer at the transmitter to join the signals together and a de-multiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as a so called optical add-drop multiplexer (OADM). Older, or fixed, OADMs cannot configure capacity at a node. The technology called Reconfigurable Optical Add/Drop Multiplexer (ROADM) represents a real breakthrough for WDM networks providing the flexibility and functionality required in today's complex networking environment. In contrast to OADMs, ROADMs allow service providers to reconfigure add and drop capacity at a node remotely, e.g. to define channels branching-off, reducing operating expenses by eliminating the time and complexity involved in manual reconfiguration. Furthermore, ROADMs allow the service provider to design an optical network once, and then never to have to worry about it again, regardless of how the network grows. The design phase in a ring topology ensures that any node can communicate with any other node using any wavelength. ROADM functionality is part of the solution for a deployable system, but in order to exploit the benefits of ROADM in metro WDM networks increased data management capabilities on individual wavelengths are also needed. (Metro network is WDM network with a range of 40-500 kms). For instance, ROADM rings are very sensitive to topology changes, and need close monitoring and control of wavelength power to keep the system in balance. The real innovation lies in the system engineering related to the ROADM function, addressing per-wavelength power measurement and management, and per-wavelength fault isolation. Almost every optical system vendor has ROADM with wavelength monitoring functions in commerce.
The next step for fully reconfigurable WDM optical networks are the tunable Small Form-factor Pluggable (SFP), where the wavelength allocation is changed as the needs of the network change. The other innovations are the tunable dispersion compensation elements. These network elements are ready-made products and can be purchased nowadays. The evolution of the optical networks seems to tend towards a fully reconfigurable network where the control and management plane will have new functions such as determining the signal quality, tuning the wavelength frequency, setting dispersion compensation units and by using Variable Optical Attenuators (VOA) setting the channel powers. Of course the traditional functions such as routing will remain its main function.
In an all-optical network the problem of routing and wavelength assignment (RWA) is critically important for increasing the efficiency of wavelength-routed. Given the physical network structure and the required connections, the RWA problem is to select a suitable path and wavelength among the many possible choices for each connection so that no two paths sharing a link are assigned to the same wavelength.
Solutions are known for this type of technology like a non-disruptive lightpath routing described in patent application U.S. 2004/0109683 A1, that presents the possibility of modifying the routing lightpaths without interrupting service.
Another solution is described in patent publication WO 2006/000510 A1 relating to optical path feasibility in an optical communication network presenting a signal quality calculation method based on physical impairments.
In both publications mentioned above, the power of certain channels within a fiber is set uniformly to equal levels. This is one of the remaining effects of the point-point optical networks. Of course there is technical simplification using this kind of channel power allocation: Using the same channel powers the nonlinear effects will have the smallest impact onto the signal quality.
This allocation schemes leads to the problem that in many cases where a channel is dedicated to a connection for a short distance due to the same channel powers the signal quality will be unnecessarily good i.e. the channel is over engineered
Additionally, in patent specification U.S. Pat. No. 7,068,932 a method and system for automatic initialization of an optical network are provided. A network management system (NMS) performs remote determination of span losses and sets the operating points of network components. The initialization method comprises remotely and automatically setting target gains of optical amplifiers and signal power levels at transmitters and receivers to required operating values. The methods for initialization of the optical network of the embodiments include gain excursion minimization (GEM) for individual channels passing through amplifiers and/or pre-emphasis of the optical link, where channel powers at the transmitters are biased to compensate for the effects of optical amplifiers gain ripple. U.S. Pat. No. 7,068,932 does not describe a network-wide optimum taking into account of constrains of links and nodes in the network.
As the size of the network that can be reached all-optically is a function of the signal power, it would be possible to use higher signal powers that would increase the size of the all optical networks, i.e. would decrease the numbers of optical-electrical-optical conversions without increased physical impairments.
It is therefore seen to be desirable to construct a method for an all-optical network where it is allowed to tune the signal power of individual channels in order to achieve an increased size of the whole network in which the quality of signals remains within an acceptable level.